1. Field of the Invention
The present invention relates to an optical recording medium on which an energy beam such as a laser beam as an optical signal is applied to record information of images, audio information or computer data, more particularly a magneto-optical recording medium including a low thermal conductivity dielectric layer which provides a high sensitivity to a recording laser power and allows a low power and short wavelength laser beam to be used.
2. Description of the Related Art
Allowing a high recording density and a large information capacity, optical recording media have been investigated and developed in various aspects and some of them have been practically realized. Among others, a magneto-optical recording medium is known. A typical magneto-optical recording medium comprises, on a transparent substrate, a recording layer of a rare earth-transition metal amorphous alloy having an easy magnetization axis vertical to the layer surface.
The recording layer of a rare earth-transition metal amorphous alloy alone has problems such as low durability and insufficient C/N ratio (carrier information/noise ratio) in reproducing. Some solutions have been proposed to improve the above properties of the amorphous alloy recording layer. One of them provides a recording medium of a four layer structure comprising a first dielectric layer, a recording layer, a second recording layer and a metal reflecting layer on a substrate.
In these optical recording media, an energy beam such as a laser beam is applied so as to record information and the laser source often used is a semiconductor laser having a wavelength of 780 nm providing an intense energy in a small volume.
Heretofore, the so-called mark position recording method has been widely adopted which comprises formation of recording marks different in an optical characteristic in the medium, the recording marks being used to represent digital information, "0" or "1". However, along with the increase in information, an increase in recording capacity of an information recording medium is required. To attain this, use of a laser beam with a shorter wavelength, so as to shorten the length of the recording marks, to shorten the space between recording marks and to narrow the pitch between neighboring recording tracks, is required.
Replacing the mark position recording method by the mark edge recording method in which the both ends of each mark are made to correspond to the "0" and "1" of the information and which allows an about 1.5 fold increase in recording capacity has also been considered. Alternatively, so-called magnetic superresolution method has also been proposed. In this method, the recording layer used has a multi-layer structure comprising writing and reading sublayers and optionally an intermediate sublayer and others, and the difference in their thermo-magnetic property is utilized, where marks on the writing layer are partially magnetically masked by the reading layer to make finer marks readable.
Thus, increase in recording capacity or recording density requires that small marks be definitely recorded at an accurate location with a predetermined size. That is, when recorded with identical laser power, linearity between the width of the recording pulse and the length of the marks must be excellent; when recorded with an identical pulth width, the dependency of the length of the marks on the recording laser power must be small; and thermal interference between adjacent marks must be small.
Particularly in the mark edge recording method, the edges of the marks represent the information and, therefore, the accuracy of the location of the edges of the marks is severely required and the thermal interference between adjacent marks becomes an important problem. Thus, it is important to control the distribution of heat generated in the recording layer by a laser beam, in other words, the diffusion of the heat must be suppressed. It is easily consieved that the dielectric layers sandwiching the recording layer and the reflecting layer be made completely thermally insulating so that the heat is forced to act only on the recording layer. In this solution, however, prevention of the heat diffusion in the stack direction results in acceleration of the heat diffusion in the layer direction within the recording layer. Further, the recording layer is deteriorated by the heat stored there and the life of the recording medium may be shortened. It is therefore desired to control the heat transfer in both the stack and layer directions to an optimum degree.
Also in the magnetic superresolution method, reading information from narrow masked regions requires precision of the location and shape of the marks. In order to effectively utilize differences in the thermo-magnetic properties of each layer of a multi-layer structure, a medium is desired in which the temperature can easily rise to the recording temperature, but in which it will not readily rise further and the mark will not expand outward, is also desired.
It is therefore an object of the present invention to solve the above problems and provide a large capacity information optical recording medium which can be used with a shorter wavelength laser source and on which mark edge recording can be utilized.